program DemoFiles; var F:TEXT; begin assign(f,'FileName.TXT'); rewrite(f); writeln(f,'Hello there'); close(f); end.The file variable "f" is used in this Pascal example in much the same way that a file handle is used in an assembly language program - to gain access to the file that was created in the program.
Function (ah): 3Dh Entry parameters: al- file access value 0- File opened for reading 1- File opened for writing 2- File opened for reading and writing ds:dx- Point at a zero terminated string containing the filename. Exit parameters: If the carry is set, ax contains one of the following error codes: 2- File not found 4- Too many open files 5- Access denied 12- Invalid access If the carry is clear, ax contains the file handle value assigned by DOS.A file must be opened before you can access it. The open command opens a file that already exists. This makes it quite similar to Pascal's Reset procedure. Attempting to open a file that doesn't exist produces an error. Example:
lea dx, Filename ;Assume DS points at segment mov ah, 3dh ; of filename mov al, 0 ;Open for reading. int 21h jc OpenError mov FileHandle, axIf an error occurs while opening a file, the file will not be opened. You should always check for an error after executing a DOS open command, since continuing to operate on the file which hasn't been properly opened will produce disastrous consequences. Exactly how you handle an open error is up to you, but at the very least you should print an error message and give the user the opportunity to specify a different filename.
axregister. Typically, you should save this value away somewhere so you can use it when accessing the file later on.
Function (ah): 3Ch Entry parameters: ds:dx- Address of zero terminated pathname cx- File attribute Exit parameters: If the carry is set, ax contains one of the following error codes: 3- Path not found 4- Too many open files 5- Access denied If the carry is clear, ax is returned containing the file handleCreate opens a new file for output. As with the OPEN command,
ds:dxpoints at a zero terminated string containing the filename. Since this call creates a new file, DOS assumes that you're opening the file for writing only. Another parameter, passed in
cx, is the initial file attribute settings. The L.O. six bits of
cxcontain the following values:
Bit Meaning if equal to one 0 File is a Read-Only file 1 File is a hidden file 2 File is a system file 3 File is a volume label name 4 File is a subdirectory 5 File has been archivedIn general, you shouldn't set any of these bits. Most normal files should be created with a file attribute of zero. Therefore, the
cxregister should be loaded with zero before calling the create function.
axregister contains the file handle for this file.
Function (ah): 3Eh Entry parameters: bx- File Handle Exit parameters: If the carry flag is set, ax contains 6, the only possible error, which is an invalid handle error.This call is used to close a file opened with the Open or Create commands above. It is passed the file handle in the
bxregister and, assuming the file handle is valid, closes the specified file.
Function (ah): 3Fh Entry parameters: bx- File handle cx- Number of bytes to read ds:dx- Array large enough to hold bytes read Exit parameters: If the carry flag is set, ax contains one of the following error codes 5- Access denied 6- Invalid handle If the carry flag is clear, ax contains the number of bytes actually read from the file.The read function is used to read some number of bytes from a file. The actual number of bytes is specified by the
cxregister upon entry into DOS. The file handle, which specifies the file from which the bytes are to be read, is passed in the
ds:dxregister contains the address of a buffer into which the bytes read from the file are to be stored.
axregister contains the number of bytes actually read. Unless the end of file (EOF) was reached, this number will match the value passed to DOS in the
cxregister. If the end of file has been reached, the value return in
axwill be somewhere between zero and the value passed to DOS in the
cxregister. This is the only test for the EOF condition.
mov ah, 3dh ;Open the file mov al, 0 ;Open for reading lea dx, Filename ;Presume DS points at filename int 21h ; segment. jc BadOpen mov FHndl, ax ;Save file handle LP: mov ah,3fh ;Read data from the file lea dx, Buffer ;Address of data buffer mov cx, 1 ;Read one byte mov bx, FHndl ;Get file handle value int 21h jc ReadError cmp ax, cx ;EOF reached? jne EOF mov al, Buffer ;Get character read putc ;Print it jmp LP ;Read next byte EOF: mov bx, FHndl mov ah, 3eh ;Close file int 21h jc CloseErrorThis code segment will read the entire file whose (zero-terminated) filename is found at address "Filename" in the current data segment and write each character in the file to the standard output device using the UCR StdLib
putcroutine. Be forewarned that one-character-at-a-time I/O such as this is extremely slow. We'll discuss better ways to quickly read a file a little later in this chapter.
Function (ah): 40h Entry parameters: bx- File handle cx- Number of bytes to write ds:dx- Address of buffer containing data to write Exit parameters: If the carry is set, ax contains one of the following error codes 5- Accessed denied 6- Invalid handle If the carry is clear on return, ax contains the number of bytes actually written to the file.This call is almost the converse of the read command presented earlier. It writes the specified number of bytes at
ds:dxto the file rather than reading them. On return, if the number of bytes written to the file is not equal to the number originally specified in the
cxregister, the disk is full and this should be treated as an error.
cxcontains zero when this function is called, DOS will truncate the file to the current file position (i.e., all data following the current position in the file will be deleted).
Function (ah): 42h Entry parameters: al- Method of moving 0- Offset specified is from the beginning of the file. 1- Offset specified is distance from the current file pointer. 2- The pointer is moved to the end of the file minus the specified offset. bx- File handle. cx:dx- Distance to move, in bytes. Exit parameters: If the carry is set, ax contains one of the following error codes 1- Invalid function 6- Invalid handle If the carry is clear, dx:ax contains the new file positionThis command is used to move the file pointer around in a random access file. There are three methods of moving the file pointer, an absolute distance within the file (if
al=0), some positive distance from the current file position (if
al=1), or some distance from the end of the file (if
al=2). If AL doesn't contain 0, 1, or 2, DOS will return an invalid function error. If this call is successfully completed, the next byte read or written will occur at the specified location.
cx:dxas an unsigned integer. Therefore, a single seek command cannot be used to move backwards in the file. Instead, method #0 must be used to position the file pointer at some absolute position in the file. If you don't know where you currently are and you want to move back 256 bytes, you can use the following code:
mov ah, 42h ;Seek command mov al, 1 ;Move from current location xor cx, cx ;Zero out CX and DX so we xor dx, dx ; stay right here mov bx, FileHandle int 21h jc SeekError sub ax, 256 ;DX:AX now contains the sbb dx, 0 ; current file position, so mov cx, dx ; compute a location 256 mov dx, ax ; bytes back. mov ah, 42h mov al, 0 ;Absolute file position int 21h ;BX still contains handle.
Function (ah): 1Ah Entry parameters: ds:dx- Pointer to DTA Exit parameters: NoneThis command is called "Set Disk Transfer Address" because it was (is) used with the original DOS v1.0 file functions. We wouldn't normally consider this function except for the fact that it is also used by functions 4Eh and 4Fh (described next) to set up a pointer to a 43-byte buffer area. If this function isn't executed before executing functions 4Eh or 4Fh, DOS will use the default buffer space at PSP:80h.
Function (ah): 4Eh Entry parameters: cx- Attributes ds:dx- Pointer to filename Exit parameters: If carry is set, ax contains one of the following error codes 2- File not found 18- No more filesThe Find First File and Find Next File (described next) functions are used to search for files specified using ambiguous file references. An ambiguous file reference is any filename containing the "*" and "?" wildcard characters. The Find First File function is used to locate the first such filename within a specified directory, the Find Next File function is used to find successive entries in the directory.
Offset Description 0 Reserved for use by Find Next File 21 Attribute of file found 22 Time stamp of file 24 Date stamp of file 26 File size in bytes 30 Filename and extension (zero terminated)(The offsets are decimal)
cxregister contains the search attributes for the file. Normally,
cxshould contain zero. If non-zero, Find First File (and Find Next File) will include file names which have the specified attributes as well as all normal file names.
Function (ah): 4Fh Entry parameters: none Exit parameters: If the carry is set, then there aren't any more files and ax will be returned containing 18.The Find Next File function is used to search for additional file names matching an ambiguous file reference after a call to Find First File. The DTA must point at a data record set up by the Find First File function.
mov ah, 1Ah ;Set DTA lea dx, DTA int 21h xor cx, cx ;No attributes. lea dx, FileName mov ah, 4Eh ;Find First File int 21h jc NoMoreFiles ;If error, we're done DirLoop: lea si, DTA+30 ;Address of filename cld PrtName: lodsb test al, al ;Zero byte? jz NextEntry putc ;Print this character jmp PrtName NextEntry: mov ah, 4Fh ;Find Next File int 21h jnc DirLoop ;Print this name
Function (ah): 41h Entry parameters: ds:dx- Address of pathname to delete Exit parameters: If carry set, ax contains one of the following error codes 2- File not found 5- Access deniedThis function will delete the specified file from the directory. The filename must be an unambiguous filename (i.e., it cannot contain any wildcard characters).
Function (ah): 56h Entry parameters: ds:dx- Pointer to pathname of existing file es:di- Pointer to new pathname Exit parameters: If carry set, ax contains one of the following error codes 2- File not found 5- Access denied 17- Not the same deviceThis command serves two purposes: it allows you to rename one file to another and it allows you to move a file from one directory to another (as long as the two subdirectories are on the same disk).
; Assume ES and DS both point at the current data segment ; containing the filenames. lea dx, OldName lea di, NewName mov ah, 56h int 21h jc BadRename . . . OldName byte "MYPGM.EXE",0 NewName byte "YOURPGM.EXE",0Example #2: Move a filename from one directory to another:
; Assume ES and DS both point at the current data segment ; containing the filenames. lea dx, OldName lea di, NewName mov ah, 56h int 21h jc BadRename . . . OldName byte "\DIR1\MYPGM.EXE",0 NewName byte "\DIR2\MYPGM.EXE",0
Function (ah): 43h Entry parameters: al- Subfunction code 0- Return file attributes in cx 1- Set file attributes to those in cx cx- Attribute to be set if AL=01 ds:dx- address of pathname Exit parameters: If carry set, ax contains one of the following error codes: 1- Invalid function 3- Pathname not found 5- Access denied If the carry is clear and the subfunction was zero, cx will contain the file's attributes.This call is useful for setting/resetting and reading a file's attribute bits. It can be used to set a file to read-only, set/clear the archive bit, or otherwise mess around with the file attributes.
Function (ah): 57h Entry parameters: al- Subfunction code 0- Get date and time 1- Set date and time bx- File handle cx- Time to be set (if AL=01) dx- Date to be set (if AL=01) Exit parameters: If carry set, ax contains one of the following error codes 1- Invalid subfunction 6- Invalid handle If the carry is clear, cx/dx is set to the time/date if al=00This call sets the "last-write" date/time for the specified file. The file must be open (using open or create) before using this function. The date will not be recorded until the file is closed.
|39h||-||Create Directory: Creates a new directory with the specified name.|
|3Ah||-||Remove Directory: Deletes the directory with the specified pathname. Error if directory is not empty or the specified directory is the current directory.|
|3Bh||-||Change Directory: Changes the default directory to the specified pathname.|
|45h||Duplicate File Handle: creates a copy of a file handle so a program can access a file using two separate file variables. This allows the program to close the file with one handle and continue accessing it with the other.|
|46h||-||Force Duplicate File Handle: Like function 45h above, except you specify which handle (in cx) you want to refer to the existing file (specified by bx).|
|47h||-||Get Current Directory: Stores a string containing the current pathname (terminated with a zero) starting at location ds:si. These registers must point at a buffer containing at least 64 bytes. The dl register specifies the drive number (0=default, 1=A, 2=B, 3=C, etc.).|
|5Ah||Create Temporary File: Creates a file with a unique name in the directory specified by the zero terminated string at which ds:dx points. There must be at least 13 zero bytes beyond the end of the pathname because this function will store the generated filename at the end of the pathname. The attributes are the same as for the Create call.|
|5Bh||Create New File: Like the create call, but this call insists that the file not exist. It returns an error if the file exists (rather than deleting the old file).|
|67h||-||Set Maximum Handle Count: This function sets the maximum number of handles a program can use at any one given time.|
|68h||-||Commit File: Flushes all data to a file without closing it, ensuring that the file's data is current and consistent.|
|Function # (AH)||Input Parameters||Output Parameters||Description|
|25h||-||Set Interrupt Vector: Stores the specified address in ds:dx into the interrupt vector table at the entry specified by the al register.|
|30h||-||Get Version Number: Returns the current version number of DOS (or value set by SETVER).|
|33h||Get Break Flag: Returns the status of the DOS break flag. If on, MS-DOS checks for ctrl-C when processing any DOS command; if off, MS-DOS only checks on functions 1-0Ch.|
|33h||-||Set Break Flag: Sets the MS-DOS break flag according to the value in dl (see function above for details).|
|33h||Get MS-DOS Version: Returns the "real" version number, not the one set by the SETVER command. Bits three and four of the version flags are one if DOS is in ROM or DOS is in high memory, respectively.|
|34h||-||Get InDOS Flag Address: Returns the address of the InDOS flag. This flag helps prevent reentrancy in TSR applications|
|35h||Get Interrupt Vector: Returns a pointer to the interrupt service routine for the specified interrupt number. See function 25h above for more details.|
|44h||-||Device Control: This is a whole family of additional DOS commands to control various devices. See the DOS programmer's reference manual for more details.|
|4Dh||-||Get Child Program Return Value: Returns the last result code from a child program in al. The ah register contains the termination method, which is one of the following values: 0-normal, 1-ctrl-C, 2-critical device error, 3-terminate and stay resident.|
|50h||-||Set PSP Address: Set DOS' current PSP address to the value specified in the bx register.|
|51h||-||Get PSP Address: Returns a pointer to the current PSP in the bx register.|
|59h||-||Get Extended Error: Returns additional information when an error occurs on a DOS call. See the DOS programmer's guide for more details on these errors and how to handle them.|
|5Dh||-||Set Extended Error: copies the data from the extended error structure to DOS' internal record.|